Foot-to-Foot Bicycle Crunch (tightens abs)
Pressing and resisting against each other's feet while doing this supereffective crunch makes your abs work harder.
With legs bent and feet flat on the ground, lie on your back, toe-to-toe with your partner. Press the soles of your feet against your partner's feet and lift them off the ground, so thighs are perpendicular and calves are parallel to the ground. Contract your ab muscles and lift your head and shoulders off the ground as far as comfortably possible. As you lift, pull your right knee toward your chest, extend your left leg, and twist your torso to bring your left elbow toward your right knee. (Your partner should do the same, so you are moving in sync and pressing against each other's soles at all times.) Hold for a second, then lower and repeat to the opposite side; that's one rep.
Partner Row (strengthens upper back)
You'll get an upper-back workout while your partner works her torso muscles.
Have your partner assume a plank position, balancing on forearms and balls of feet. Using good form (back straight, knees behind toes), squat down and pick up her legs. Leaning forward from your hips, bend your knees slightly, and extend your arms down at your sides. This is the start position. Keeping your back straight, abs tight, and elbows close to your body, squeeze your shoulder blades together and pull your elbows straight back, lifting your partner's body up. Your partner should contract her torso and lower-body muscles to stay straight. As you lift her, she should pivot from her shoulders, not her back or hips. Hold for a second and then slowly extend your arms back down.
Double Bridge (firms glutes, hamstrings, quads, and abs)
Your quads get an extra workout when you and your partner press the soles of your feet together.
With legs bent and feet flat, lie toe-to-toe with your partner. Pressing the sole of your left foot against your partner's right foot, extend those legs up toward the sky. Contracting your glutes and pushing down on your grounded feet, lift your hips, so your bodies form straight lines from your bent knees to your shoulders. Keep hips level. Hold for a second and then slowly lower. Repeat for a full set; then switch legs.